A large scale in situ hybridization screen is actually performed in order to characterize all the genes whose expression is spatially regulated during the zebrafish embryogenesis. This in situ screen: 1. Provides a large collection of cell and/or tissue specific molecular markers usable for the analysis of the wild-type development (establishment of a "molecular anatomy" of the zebrafish embryo) as well as for the phenotypic description of mutants obtained from genetic screens. 2. Provides starting points for a functional analysis (using overexpression studies by RNA injection). 3. Provides a powerful way to clone genes disrupted in mutant embryos based on the correlation between the mutant phenotypes, the expression pattern, sequence and map position of these genes. 4. All these expression data will be released to public in an integrated database (ZFIN) in a standardized format allowing search both by name, sequence, genetic map, anatomical structures and will include a description using key words of all spatially restricted expression patterns as well as the corresponding pictures for all analyzed developmental stages.